Disable HTTP/2

HTTP/2 support is golang has many problematic cornercases where dead
connections would be kept.

golang/go#32388
golang/go#39337
golang/go#39750

I suggest we disable HTTP/2 for now and enable it manually on the
blackbox exporter.

Signed-off-by: Julien Pivotto <roidelapluie@inuits.eu>

config/http_config.go

@@ -29,7 +29,6 @@ import (
 	"time"
 
 	"github.com/mwitkow/go-conntrack"
-	"golang.org/x/net/http2"
 	"gopkg.in/yaml.v2"
 )
 
@@ -154,11 +153,6 @@ func NewRoundTripperFromConfig(cfg HTTPClientConfig, name string, disableKeepAli
 				conntrack.DialWithName(name),
 			),
 		}
-		// TODO: use ForceAttemptHTTP2 when we move to Go 1.13+.
-		err := http2.ConfigureTransport(rt.(*http.Transport))
-		if err != nil {
-			return nil, err
-		}
 
 		// If a bearer token is provided, create a round tripper that will set the
 		// Authorization header correctly on each request.